Overview
You should be able to:
- Describe current as a flow of charge and use Q = I t.
- Use and explain series and parallel circuits, and how to place ammeters and voltmeters.
- Use V = I R, P = I V, E = I V t and E = Q V.
- Understand resistance, LDRs, thermistors and I–V characteristics.
- Explain mains vs d.c. from cells, and how fuses, insulation and earthing keep us safe.
- Describe basic electrostatics: charging by friction, attraction/repulsion, dangers and uses.
Electricity questions often mix skills: for example, reading a circuit diagram, using V = I R and then picking the correct fuse from P = I V.
1. Current & Charge
What is Current?
Electric current is how much charge passes a point each second. In a metal wire this charge is carried by electrons.
Unit of current: ampere (A)
Unit of charge: coulomb (C)
Q = I t
1 C = 1 A × 1 s → if 2 A flows for 3 s, then 6 C of charge has passed.
Charge Flow in a Wire
Electrons move opposite to the direction of conventional current.
Quick Check
Q1. A current of 3 A flows for 5 s. Calculate the charge that passes.
Show answer
Q = I t = 3 × 5 = 15 C
Q2. 24 C of charge passes in 8 s. What is the current?
Show answer
I = Q / t = 24 / 8 = 3 A
2. Circuits, Series & Parallel
Series vs Parallel
- Explain why a series or parallel circuit is more appropriate for a particular application.
- Know that lamps and LEDs can show if there is a current in a circuit.
- Know that the voltage across components connected in parallel is the same.
- Understand why current is conserved at a junction in a circuit.
Measuring Current & Voltage
- An ammeter is connected in series and has very low resistance.
- A voltmeter is connected in parallel across a component.
At a junction in a parallel circuit: total current in = total current out.
Quick Check
Q3. In a parallel circuit, 2 A flows in the main wire. One branch takes 0.8 A and another takes 1.2 A. Is current conserved at the junction?
Show answer
Yes. 0.8 A + 1.2 A = 2.0 A → current in = current out.
3. Voltage, Energy & Power
Potential Difference (Voltage)
V = E / Q
1 volt = 1 joule per coulomb (1 V = 1 J/C).
E = Q V
Power & Mains Safety
P = I V
Power is how quickly electrical energy is transferred. Unit: watt (W).
E = I V t
You should understand how insulation, double insulation, earthing, fuses and circuit breakers protect the user and device from dangerous currents, and how P = I V helps you select the correct fuse rating.
Also know the difference between mains electricity (a.c.) and direct current (d.c.) from a cell or battery.
Quick Check
Q4. A kettle is rated 2 000 W at 230 V. What current does it use?
Show answer
I = P / V = 2 000 / 230 ≈ 8.7 A → a 13 A fuse would be suitable.
Q5. A 0.5 A current flows through a 12 V lamp for 10 min. How much energy is transferred?
Show answer
Convert time: 10 min = 600 s.
E = I V t = 0.5 × 12 × 600 = 3 600 J.
4. Resistance & I–V Characteristics
Ohm’s Law
V = I R
Unit of resistance: ohm (Ω).
In a series circuit the current is the same everywhere, and depends on the applied voltage and the number and nature of the components.
Increasing resistance (e.g. adding more resistors) reduces the current.
I–V Graphs
You should be able to describe how current varies with voltage for:
- a metal wire / resistor at constant temperature,
- a filament lamp,
- a diode.
LDRs & Thermistors
You should be able to describe qualitatively how resistance changes for:
- LDR (light-dependent resistor) – resistance decreases when light intensity increases.
- Thermistor – resistance decreases when temperature increases (for NTC thermistors).
Quick Check
Q6. A 12 V supply drives a current of 0.3 A through a resistor. What is its resistance?
Show answer
R = V / I = 12 / 0.3 = 40 Ω
Q7. In a dark room an LDR has high resistance. What happens to the current through it when the room becomes bright?
Show answer
Its resistance falls, so (for the same voltage) the current increases.
5. Electrostatics
Charges & Materials
- Identify common materials which are electrical conductors or insulators (metals, plastics, etc.).
- Describe and investigate how insulating materials can be charged by friction.
- Explain how positive and negative electrostatic charges are produced by the loss and gain of electrons.
- Know there are forces of attraction between unlike charges and forces of repulsion between like charges.
Dangers & Uses
- Explain the potential dangers of electrostatic charges, e.g. when fuelling aircraft and tankers.
- Explain some uses of electrostatic charges, e.g. in photocopiers and inkjet printers.
Quick Check
Q8. A plastic rod is rubbed with a cloth and becomes negatively charged. What has happened at the particle level?
Show answer
Electrons have moved from the cloth onto the rod; the rod gains extra electrons → negative charge.
What Next?
Once you are confident with this page, practise with past-paper questions on:
- Q = I t, V = I R, P = I V and E = I V t / E = Q V.
- Series and parallel circuits, including correct placement of ammeters and voltmeters.
- Resistor, filament lamp and diode I–V graphs, and LDR/thermistor behaviour.
- Electrostatic charging, dangers and applications.